The present invention relates to composite video decoders, and more particularly to a diagonal correction method in composite video decoders for reducing errors in recovered luminance and chrominance signals due to diagonal high frequency luminance information being present in an encoded video signal.
In a displayed image where there is diagonal high frequency luminance information in an encoded video signal, errors occur in the decoding which appear as disturbing rainbow patters together with a loss of luminance contrast. This phenomenon commonly is known as "cross color." These cross color errors in decoding are caused by high frequency luminance information being misinterpreted as being partially chrominance. Due to the subcarrier phase reversal between lines and fields in the encoded video signal, the polarity of the chrominance errors reverses from frame to frame, or picture image to picture image. This makes the chrominance error quite visible to a viewer.
To better understand the problem of cross color it is helpful to look at an extreme case. Although the problem and the solution of the present invention are described herein in terms of NTSC encoded video, the discussion is equally applicable to PAL encoded video by accounting for the PAL differences from NTSC. In a video encoder red, green and blue video signals are combined to form a luminance signal and two color difference signals, the difference signals together describing the chrominance. The chrominance signals are low pass filtered to less than 1.5 MHz. The encoder produces two sine waves with a ninety degree phase difference at what is known as the subcarrier frequency, approximately 3.58 MHz for NTSC and 4.43 MHz for PAL. Each of the color difference signals amplitude modulates one of the sine waves, and the modulated sine waves are summed together to produce a single sine wave at the subcarrier frequency. This process is commonly referred to as quadrature amplitude modulation (QAM). The phase of the sine wave with reference to the subcarrier carries hue information, and the amplitude describes the saturation. The luminance information is added to the QAM chrominance signal to provide the brightness signal in the encoded signal.
Keeping the encoding process in mind the extreme case of the cross color problem is described. Given a video signal having alternating black and white lines at the subcarrier frequency and at an angle of forty-five degrees as shown in FIG. 1 with the black and white lines exchanging positions every frame, the encoder does not produce any chrominance information because none is present, i.e., the QAM chrominance signal has a zero amplitude. The encoder then adds in the luminance information to produce the encoded signal. When a decoder receives the signal energy at or near the subcarrier frequency, it interprets it as chrominance. In this extreme example all of the energy in the signal is at the subcarrier frequency and therefore is decoded as chrominance information. Also the decoder produces a luminance signal indicating a constant brightness rather than black and white lines. Thus the picture which is displayed, which should appear as black and white diagonal lines without color, appears as a constant color without lines. Even a comb filter decoder produces the same erroneous display due to the angle of the lines. A frame comb filter also produces the same display because the lines exchange position every frame. Therefore the encoded video signal having luminance information only is decoded as chrominance signal having no contrast in this extreme case.
It has been suggested that the best way to eliminate cross color in the decoded image is to pre-filter the luminance information in the video encoder before it is combined with the chrominance information. The filtering reduces or eliminates high frequency diagonal luminance information in the encoded signal, but at the expense of reducing or eliminating picture detail. Further this does not address the problem of all the video material already encoded by standard video encoders and recorded on videotape.
Another suggestion is the use of frame comb decoding, but such schemes do not work in the portions of the image depicting objects in motion.
In practice, however, cross color errors are not so overwhelming as in the extreme case described above, but it is annoying where and when it occurs. The magnitude of the error in any given situation increases as the frequency of the luminance information approaches the subcarrier frequency and as the angle approaches forty-five degrees from the horizontal, the angles being multiple and different in the PAL system. The error also increases with the contrast within the luminance information.
What is desired is a diagonal correction method in a composite decoder which effectively reduces the errors in the recovered luminance and chrominance signals caused by high frequency diagonal luminance information in an encoded video signal without the requirement of non-standard video encoders and regardless of the motion within a picture.